Most copper ores or concentrates from which copper is extracted on a commercial scale contain copper in sulphide form, such as CuFeS.sub.2 (chalcopyrite), Cu.sub.5 FeS.sub.4 (bornite) and Cu.sub.2 S (chalcocite) (hereinafter referred to as "sulphide ores"), as opposed to copper in oxide form (hereinafter referred to as "oxide ores").
Extraction processes for copper may be broadly classified into two categories, i.e. smelting and leaching. In general, smelting processes are applied to sulphide ores, whereas leaching is more often used with oxide ores. It is noted that the preliminary step of concentration of ores (by flotation) is usually applied to sulphide ores, not oxide ores.
The reasons for the differing treatment for the two types of ores are generally technical, and thus also economical. Sulphide ores float well, producing relatively high grade concentrates from low grade ores. These sulphide concentrates are well suited to the proven smelting process. Oxide ores on the other hand, do not concentrate easily and therefore it is difficult to provide a feed material that is sufficiently high grade for smelting. In addition, oxide ores do not have any natural fuel, comparable to the sulphur in sulphide concentrates. Fortunately though, oxide ores do leach easily in sulphuric acid solutions, and thus a sizeable industry has been established based on heap leaching of oxide ores, followed by solvent extraction and electrowinning.
Despite all of the above, smelting of sulphide ores has some serious drawbacks, mostly concerned with the need to avoid air pollution due to the sulphur gases emitted. In locations where there is no market or use for sulphuric acid, it is very difficult to justify a smelter on economic grounds, assuming it is not permitted to vent the gases freely to the atmosphere. Copper mines which produce sulphide concentrates in such locations usually ship the concentrates to distant (offshore) smelters. When smelting/refining/shipping charges are low, this may be economic, but there are times when the available world smelting capacity for custom concentrates is limited and, consequently, smelting charges rise sharply according to the law of the marketplace. At such times, concentrate producers may be placed at a severe disadvantage.
Therefore there is a need for an economical and technically sound leaching process that can treat sulphide concentrates at the mine site, particularly concentrates based on chalcopyrite, as this is the most widely distributed copper mineral worldwide.
One of the problems which arises with the leaching of copper from sulphide ore is that, while a high acid concentration may be desirable during the leaching stage, such a high concentration is undesirable during the subsequent solvent extraction stage, due to unfavourable equilibrium conditions which are created and leading to higher equipment and working costs. This requires neutralization in order to yield a pregnant liquor suitable for solvent extraction, rendering the process uneconomical, in particular where low grade ores are involved.
It is accordingly an object of the present invention to alleviate the above-mentioned difficulties.